The worms, dubbed nematodes after the parasitic worm used to kill pests in gardens, could give security administrators the ability to scan machines inside a corporate network but beyond a local subnet, David Aitel, principal researcher of security firm Immunity, said at the Black Hat Federal conference.

"Rather than buy a scanning system for every segment of your network, you can use nematodes to turn every host into a scanner," he said during an interview with SecurityFocus. "You'll be able to see into the shadow organisation of a network - you find worms on machines and you don't know how they got there."

Later attempts at creating 'good' worms have failed, however, mainly because the writers have not adopted many of the safeguards outlined in the Bontchev paper. The Welchia worm - a variant of the MSBlast, or Blaster, worm - had apparently been created to fix the vulnerability exploited by the MSBlast worm, but had serious programming errors that caused the program to scan so aggressively for new hosts, it effectively shut down many corporate networks.

Immunity's research is the latest attempt to create a more rigorously conceived framework for creating worms that could spread across specific networks to find and report vulnerabilities. The research essentially offers two advances, a strategy for the controlled propagation of worms and a framework in which reliable worms could be created quickly, Aitel said.

"History has repeatedly shown us that people who write worms by hand make mistakes," he said. "Worms are difficult to build and very difficult to test."

The nematode worms would have to get permission to spread by querying a central server for a specific digital token, which Aitel dubbed a nematoken, before spreading to a particular machine. Another version of the software would use a whitelist to spread among only the company's computers.

Because the worms would be limited to spreading in a specific company's network, they would be completely legal, said Aitel. He noted that penetration testers today are given the right by a company to exploit systems on that company's networks. The distributed nature of the worms do make ascertaining permission more difficult, he acknowledged.

Aitel's idea is a new twist on an old concept. An author using the name MidNyte wrote a response to Bontchev's paper in 1999 arguing that a 'good' virus that kept information on the last 100 hosts to which it spread could help defend against bad viruses.

However, Aitel also argues that, in today's complex networks, nematodes could significantly reduce the cost of scanning a large network, by bringing the advantages of peer-to-peer concepts to penetration testing and network scanning. Rather than buying a new sensor for each subnet in a company, the nematode could spread using existing pathways to enumerate any computers with a given set of vulnerabilities. Moreover, the technology could be used to move search agents across a network to find specific files or to push intelligence to all desktops without a specific client.

On the other hand, the dangers inherent in self-propagating code are hard to overcome, said Jose Nazario, senior security and software engineer for network defense firm Arbor Networks.

"I still have my doubts that the controls he described are effective enough," Nazario said. "He addressed how you shut the nematodes down and how you make sure they don't infect other networks, but he hasn't addressed machine instability and the danger when people carry laptops across network boundaries."

Nazario, the author of Defense and Detection Strategies Against Internet Worms, believes the best way to find vulnerabilities on a large network is to use dedicated sensors, an approach used by Arbor Networks.

"There are a number of ways of finding those vulnerabilities in the network without the inherent risks involved in self-propagating code," he said.